Such a plug connector is disclosed by EP 0590496 A1. The plug connector exhibits a contact part holder which is connected in one piece to the housing frame. A further contact part holder is constructed as a separate module and can be inserted into a hollow space in the housing frame in the plug-in direction of the contact parts parallel to the contact part holder fixed to the housing. In order to reduce the plugging forces when connecting the frequently multi-pole plug connector to the mating connector the plug surfaces of the two contact part holders are arranged in different planes so that a steplike, force-spreading connection ensues. The initial locking of the contact parts takes place by means of flexible fingers which project from a wall of the contact part cavity in question and catch in the contact part. After the complete plug connection has been produced pins arranged on the mating connector serve as blocking members for the flexible fingers. It is disadvantageous that there is no active secondary locking and that due to the aforementioned contact part locking the correct seating of the contact parts in the contact part cavities is only checked during connection to the mating connector. Incorrect or incomplete positioning requires a costly correction. Contact parts, especially contact pins, which project beyond the plug-in plane are protected only inadequately before and during fitting to the mating connector by a salient cup-shaped elongation of the housing wall. Individual contact parts can be mechanically damaged and give rise to repairs or malfunctions. Sealing of the individual units is not discernible.
DE 3526664 C2 discloses a plug connector in the housing of which a separate contact part holder is inserted. The contact parts are introduced into the contact part holder and provided with secondary locking by means of a swivelling, hinged flap. The contact part holder is then inserted into the housing and fixed in detachable manner with a flexible cantilever arm. No provision is made in a housing of this type for the integration of further contact part holders or contact part holders fixed to the housing. The construction of a modular system is not taken into consideration. Here also contact parts protruding from the plug-in plane are not protected against mechanical damage prior to fitting to the mating connector.